tan(x1+x2)/2=sin(x1+x2)/1+cos(x1+x2)怎么算出来的

Question

tan(x1+x2)/2=sin(x1+x2)/1+cos(x1+x2)怎么算出来的

Answer 1

记x1+x2=a
sin(x1+x2)/1+cos(x1+x2)=sina/(1+cosa)=2sin(a/2)cos(a/2)/(2cosa^2)
=tan(a/2)=tan(x1+x2)/2